1. Field of the Invention
The present invention relates generally to the field of electric discharge machining and more particularly to an improvement in apparatus for abrasively forming EDM electrodes wherein means are provided for sensing the initial contact between the abrading die and electrode workpiece.
2. Description of the Prior Art
Prior to the present invention a method and apparatus for forming EDM electrodes were disclosed in U.S. Pat. Nos. 3,663,785 entitled Method of Erosively Shaping a Master Die and U.S. Pat. No. Re. 27,588 entitled Apparatus for Shaping Electrodes, respectively. An improved apparatus was described in U.S. Pat. No. 4,186,527 which issued on Feb. 5, 1980.
The basic method and apparatus for its implementation involve the use of an abrasive die and an electrode workpiece mounted in working alignment with one another. The die and workpiece are moved relative to one another in a horizontal oscillatory rotary motion as they are brought together in the vertical direction such that the workpiece is abrasively formed into the same shape and contour as the die. The later patent describes an improvement wherein the relative vertical motion of the die and workpiece is automatically controlled as a function of the relative vertical position thereof.
Even with the improved vertical motion control arrangement it remains necessary to initially position the die with respect to the workpiece in the vertical dimension. Prior to the present invention this initial positioning was accomplished manually. The operator would cause the vertically movable platen to infeed until he visually determined that the die and workpiece came into physical contact. The platen would then be caused to outfeed a preselected distance and the cyclic vertical motion initiated after which it would be controlled automatically.
The visual determination of initial contact between the die and workpiece has been found by the applicant to be imprecise at best, being of variable accuracy from operator to operator. As a result, the otherwise high degree of accuracy obtainable with the automatic control arrangement was impaired through initial set-up errors. The same problem existed in the initial set-up of the earlier timer controlled abrading machines.
One potential solution to the set-up problem was to use a pressure sensing device to indicate contact, however, mounting such a sensor in a manner that will not interfere with the abrading process and yet compatible with a high level vibration environment is complex and expensive.